The present invention relates generally to a semiconductor radiation detector operable in room temperature range and with a low supplying voltage for detecting radioactive rays (hereinafter also referred to as radiation). In particular, the present invention relates to an improved structure of a semiconductor radiation detector which is significantly enhanced in respect to the detection sensitivity and the capability of maintaining a constant sensitivity independent of variations in energy level of the incident radiation (hereinafter also referred to as the energy compensating capability).
There is a known semiconductor radiation detector having a structure in which a layer of a fluorescent material is deposited on a detector element, and wherein light emitted by the fluorescent layer in response to impingement of the radiation is guided to a depletion layer formed internally of the semi-conductor detector element, thereby assuring an enhanced or increased detection sensitivity for the radiation of low energy level (lower than 10 KeV), as is disclosed in Japanese Patent Application Laid-Open No. 55075/1984 (JP-A-59-55075).
Usually, the range of energy to be measured by the ordinary radiation monitors and dosimeters or dosemeters is of 80 KeV to 3 MeV, as recommended in Japanese Industrial Standards (JIS) Z4324 P450. Accordingly, with the range of energy envisaged by the radiation detector disclosed in the above cited publication, use of the radiation detector is apparently restricted to a specific application. Further, endeavours made heretofore in an effort to increase the detection sensitivity per unit area of the semiconductor radiation detector are almost exclusively concerned with how to increase the thickness of a depletion layer formed in the radiation sensitive portion, as is described in Japanese Patent Application Laid-Open No. 108367/1984 (JP-A-59-108367). In this connection, it is known that the thickness of the depletion layer as formed is proportional to the 1/2-th power of the voltage supplied to the semiconductor detection element. As is also disclosed in the publication mentioned just above, that sensitivity of the radiation detector element can be changed by varying the voltages supplied to a main electrode and an annular control electrode. Thus, the known arrangement is based on the concept of varying the sensitivity of the radiation detector element by resorting exclusively to the factors external of the detector element, which is equivalent to the employment of two detector elements for the purpose of doubling the sensitivity.
As will be understood from the above, approaches for increasing the detection sensitivity in the hitherto known semiconductor radiation detectors rely on the external means such as the provision of the fluorescent film, control of the supplied voltage and so forth.
On the other hand, Japanese Patent Application Laid-Open No. 74375/1987 (JP-A-61-74375) discloses a structure of the semiconductor radiation detector in which a p-n junction is formed in such a pattern that a depletion layer makes appearance in a radial configuration having a plurality of elongated regions extending radially, and which is also provided with a region extending around the depletion layer on a semiconductor substrate surface, the region having a width corresponding to the range of the secondary electrons produced by the incident gamma rays having the maximum energy level of those to be detected, wherein the secondary electrons produced by the gamma rays impinging on the region enhance the energy compensating capability.
Of the known techniques described above, the semiconductor radiation detector of the structure in which the fluorescent film is deposited presents a problem from the stand point of practical application that the range of energy to be measured by the ordinary radiation monitor and the dosimeter or dosemeter (as recommended by JIS) is not taken into consideration in the use of the radiation detector. On the other hand, the semiconductor radiation detector in which the applied voltage is changed suffers a disadvantage that the exposure dose rate can not be determined with acceptable accuracy because no consideration is paid to the fact that the energy compensating capability (i.e. the capability of maintaining the detection sensitivity to be constant independent of energy level of the gamma rays incident on the detector) will change when the thickness of the depletion layer varies. Certainly, the last mentioned prior art is concerned with the improvement or enhancement of the energy compensating capability. However, since no attention is paid to a leakage current flowing through the semi-conductor substrate, it is expected that some kind of problem will arise in the practical application, as well as because of the absence of the means for increasing the detection sensitivity of the radiation detector.